It depends in which context you're asking this question, diploma courses can add value to one's CV depending upon the skills of domain of work, like a lot of people go for diploma courses in foreign language, computer courses(software as well as hardware), courses that are relevant in their professional world

If you're specifically referring to the diploma and degree programme in engineering, then yes, degree courses are always valuable, but then a lot of people for various reasons cannot go ahead with degree engineering program, and they choose to do diploma, now the question is it worth it? Well, it depends on a lot of factor the institute one enrolls into, the placement opportunities and various other facilities that an institute ought to provide for overall development of a student which is necessary in this day and age. Diploma engineering is also a stepping stone for further studies, you can go ahead with degree programme after successful completion of diploma engineering and work opportunities are more once you go for higher studies for these students.